Systems delivering dialysis solution to a hemodialysis unit are of three major types.
Batch type systems utilize a given volume of prepared dialyzing fluid in a tank. Fresh batches of fluid must be made as the fluid's waste removal capacity is exhausted. Where a small tank is used or where several dialyzers are supplied from the same tank, it is occasionally necessary to mix up two or more batches of solution before a dialysis run is complete.
Sorbent regenerative systems utilize a smaller volume of fluid which is cycled through a tube of absorbent material to remove waste from the fluid.
Proportioning systems continuously mix and deliver fresh fluid to the dialyzer. Such systems utilize a concentrate prepared in bulk which is then mixed in a predetermined ratio with water. Metering pumps and water-powered cylinders of fixed stroke are typically used to provide the correct proportion of water to concentrate. Electronic feedback systems, utilizing a conductivity meter to control a concentrate pump, have also been devised.
For batch or sorbent systems, it is possible to prepare dialysis solutions directly from dry reagents. This is unusual, however, since such preparation requires careful weighing of a number of ingredients, typically including sodium chloride, sodium bicarbonate or sodium acetate, potassium chloride, calcium chloride, magnesium chloride and lactic acid. Glucose or invert sugar may be added to adjust the osmolality of the solution to a desired level. Because of this complexity, most dialysis solutions are prepared from commercially supplied concentrate.
Proportioning systems require the use of some kind of a solution concentrate.
Preparation of dialysis solutions from concentrates, however, sometimes poses problems. Shipment costs for the concentrate are substantially higher than those of dry reagent. Large drums of concentrate may stratify, with the certain salts tending to layer out at the bottom. Furthermore, acetate, rather than bicarbonate, must be used as buffer because of the instability of bicarbonate. While this is not usually a problem since acetate is metabolized by the body to produce bicarbonate, some patients, particularly those with liver disease, cannot metabolize acetate ion.